The study of genetic toxicology is receiving substantial scientific attention because of the potential major role of mutagenesis in such diverse human health problems as cancer, aging and birth defects. Research in genetic toxicology is, relatively speaking, still in its infancy. The precise implications for humans of results from these in vitro and in vivo assays are still vaguely understood. Exploration of the mechanistic role of mutagenesis in each individual health concern will be a lengthy and involved process. The need for sound, statistically based evaluation of genetic toxicity data as part of this learning process is recognized by most experts in the field. Responding to this need is, and will continue to be, the primary motivation for this project. Design and analysis of individual assay experiments, together with efforts at assay validation, are areas in which statistical methodology contributions from this project have been notable. Emphasis in project research is shifting from in vitro to in vivo assays; this is, in part, an acknowledgement that the in vivo chemical environment is closer to that of an intact mammal for purposes of risk assessment. Large databases derived from NTP and international collaborative studies continue to provide the empirical foundation upon which new statistical methodologies are constructed and evaluated.